Various imaging technique have been developed in the medical field to observe the inside of a subject to make diagnosis. In particular, ultrasound imaging which obtains internal body information of subject by transmitting and receiving ultrasound allows not only real-time observation of images but also saves the subject from exposure to radiation, unlike other medical image technique such as an X-ray photograph and a radio isotope (RI) scintillation camera etc, Therefore, as highly safe imaging technique, ultrasound imaging is used not only for a fetal diagnosis in obstetrical field but also used for wide range of areas such as the gynecologic system, the circulatory system, and the digestive system. The ultrasound imaging is an image generation technique which takes advantage of the property that the ultrasound is reflected at the boundary of regions having different acoustic impedances (e.g., a boundary of a structure). The contour of a structure that exists inside the subject (e.g., an internal organ, and diseased tissue) is extracted by transmitting the ultrasound beam to the inside of the subject such as a human body, receiving an ultrasound echo generated in the subject, and obtaining a reflecting point at which the ultrasound echo is generated and its reflection intensity.
A device which performs the ultrasound imaging (called, for example, an ultrasonic diagnostic device, and an ultrasonic imaging device) includes a transducer (piezoelectric transducer) as an ultrasonic transducer which transmits and receives ultrasound. The transducer generally used is obtained by forming electrodes on both sides of a piezoelectric body such as a piezoelectric ceramic represented by a Pb (lead) zirconate titanate (PZT), and a polymeric piezoelectric body material represented by a polyvinylidene difluoride (PVDF).
FIG. 2 is a schematic view showing an example of a one-dimensional array type ultrasonic probe. As shown in FIG. 2, an ultrasonic probe 403 that is of a one-dimensional array type includes: a plurality of ultrasonic transducers 205 each including a piezoelectric body 208, a signal electrode 206, a ground electrode 207, and a matching layer 203; a backing 201; a grounding line 202; signal wiring 204; and an acoustic lens 209.
All of the ground electrodes 207 are connected to the one grounding line 202. Each of the signal electrodes 206 is connected to a corresponding one of the signal wiring 204. The signal electrode 206 and the ground electrode 207 are bonded to a pair of opposing surfaces of the piezoelectric body 208. The direction toward the ground electrode 207 as seen from the signal electrode 206 is referred to as +Z. The matching layer 203 is provided on the +Z side with respect to the ground electrode 207.
As shown in FIG. 2, the ultrasonic probe 403 that is of the one-dimensional array type includes a plurality of the ultrasonic transducers 205 arranged in a one-dimensional array on the +Z side of the backing material 201 which absorbs sound waves. Furthermore, further on the +Z side with respect to the ultrasonic transducer 205, the acoustic lens 209 is provided. The ultrasonic probe 403 emits ultrasound to a subject (not shown) via the acoustic lens 209.
The voltage applied to each of the two electrodes 207 and 206 of the ultrasonic transducer 205 causes expansion and contraction of the piezoelectric body 208 due to the piezoelectric effect, and ultrasound is thus generated. It is possible to form an ultrasound beam which is transmitted to a predetermined direction, by arranging the ultrasonic transducers 205 one-dimensionally (or two-dimensionally) as described above, and driving each of the ultrasonic transducers sequentially. Furthermore, the ultrasonic transducer receives the ultrasound reflected off the inside of the body of the subject, and expands or contracts to generate an electric signal. The electric signal is used as a reception signal of the ultrasound.
FIG. 4 is a schematic view showing an example of an ultrasonic diagnostic device. As shown in FIG. 4, an ultrasonic diagnostic device 401 includes an ultrasonic probe 403. The ultrasonic probe 403 and a diagnostic device main body 404 are connected via a cable 405. The diagnostic device main body 404 transmits, via the cable 405, a signal for vibrating the ultrasonic transducer to the ultrasonic probe 403, and creates an image of internal condition of the subject as an ultrasonic diagnostic image based on the signal from the ultrasonic probe 403.
The above-described ultrasonic probe includes the matching layer 203 between the piezoelectric body 208 and the subject due to the following reason.
The propagation efficiency of the ultrasound at the interface where different substances contact each other changes depending on the acoustic impedance of the each of the substances. Specifically, the ultrasound reflects well at the interface where the difference in acoustic impedance is large, which causes large propagation loss of the ultrasound.
In view of this, a matching layer is provided between the ultrasonic transducer and the subject to match the acoustic impedance. The acoustic impedance changes in a stepwise manner through the matching layer from the transducer toward the subject. This lowers reflectance of ultrasound at each of the interfaces and thereby propagation loss of the ultrasound is reduced.
However, providing the matching layer to increase the propagation efficiency of the ultrasound is known to cause an adverse effect, that is, the frequency band becomes narrow and thus the resolution of the ultrasonic diagnostic image becomes degraded. The realization of the matching layer which does not degrade the resolution of the ultrasonic diagnostic image is required.
Conventionally, to build the matching layer which does not degrade the resolution of the ultrasonic diagnostic image, a technique has been disclosed to widen the band by providing, in a plane perpendicular to the propagation direction of the ultrasound, a plurality of regions in which the matching layers have different thicknesses (e.g., patent literature 1).